Monday, 5 October 2009

The above two shots were taken a minute apart on Saturday at Leeds Castle. The first uses purely ambient light and lacks drama - details in the sky have been lost and the subjects' faces are poorly lit. I therefore decided to liven things up with a bit of off-camera flash.

I asked the chauffeur of the Rolls Royce to hold a 580EX II Speedlite fitted with a Lastolite micro Apollo. This was set to slave mode and then triggered with an ST-E2 transmitter. The flash head was rotated so that the light could be directed at the couple and the wireless sensor at the transmitter.

Ambient light was underexposed by 2 stops and flash exposure compensation of -2/3 stop was used.

9 comments:

Mark
said...

David, I'm a bit confused with using ETTL flash with my camera in manual mode (5D2 and 580EXII). For portraits I'm killing the ambient with 160shutter and working at F5.6, but the ETTL is seriously underexposing my scene (mainly white). Is having to FEC+ normal? Just seems like the ETTL should be doing a much better job.

It is, I'm afraid - particularly when scenes are predominantly low- or high-key. It's the classic problem caused by determining any exposure using reflected light.

In the situation you're encountering, the white surfaces are reflecting back a lot of flash light and the camera is consequently under-exposing the shot. Bear in mind that the camera doesn't know what it's looking at - it can't tell that the large amount of light reflected back is due to this.

Once you understand the principles involved you'll realise why all exposure algorithms will always have limits.

I have also noticed something very strange. Now the max sync speed of the 5DII is 1/200 before you need to use HSS. However, if you have the camera in manual mode and the flash in manual, the max sync speed is reduced to 1/160 if you also have the HSS button pressed. It shouldn't really be kicking in until 1/250 right? I have no explanation for this. Have you come across it or any ideas?

If you have both the camera and Speedlite in manual then there is no minimum shutter speed with FP-mode (high speed sync) enabled (as opposed to the 1/160s that you mention).

If you weren't in FP-mode you would expect to be limited by a X-sync speed of 1/200s (not sure where 1/250s came from).

I've just verified this with a 5D Mark II and 580EX II. For me the flash sync speed is 1/200s in all modes.

With the camera in manual mode I can set a shutter speed of 1/200s (but no less) if the Speedlite is in normal mode, as opposed to FP-mode. It makes no difference if the Speedlite is set to manual or E-TTL. As soon as I switch the Speedlite to FP-mode I can access faster shutter speeds.

I put my 5D Mark II and 580EX II in manual mode. With shutter speeds slower that 1/200s it made no difference if the flash unit was in FP-mode or not. With a shutter speed of 1/200s, however, the 580EX II was adding less light when in FP-mode.

What this obviously means is that at 1/200s the Speedlite is actually using FP-mode (which reduces flash output) even though it should be able to sync fully with the shutter.

Now I'm noticing some behaviour I don't understand on my 5D2 and 580EXII: If I expose a scene in AV mode so the meter scale rests on 0 (for example), I then take the same ISO/shutter/Apperture readings into Manual mode - but if the 580EXII is on the camera EV meter now shows -1. So in AV mode the ambient exposure is always -1 stops?If you turn the flash off - AV mode will then meter ambient same as Manual mode.

E-TTL has to balance flash and ambient light and decide whether the flash serves as key or fill light. It often tends to underexpose for ambient light. The addition of flash then 'tops up' the exposure - and you now have your light where you want it!